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Comment by LanternLight83 | Hacker News Reader

Not #3, but it's still the best option

I've run a mirror with non-identical disks on ZFS, worked just like any other mirror. I bought two pairs of disks from different vendors and ran them as mixed pairs in case the disks from one vendor would fail simultaneously.

Disk space is, as expected, limited to the smaller disk in each pair. However, if you replace the smaller disk with a larger one, ZFS will automatically grow to the new minimum. You fault one disk in a pair, swap it with a larger and bring it online, ZFS will repair (fill the new disk) and once finished the pool will be larger just like that.

I've used this approach to increase the size of my pool, which again meant having a pair of different disks in the mirror as I did the the swap process.

Hmm I think you can mirror different sizes just fine but it'll predictably use the smaller size, no? Is btrfs different in that regard? I have only used it when it was released many years ago so my memory is rusty.

Btrfs' RAID1 works bit differently. It ensures every block (1GB by default I believe) is present on two devices. So, if for example, you have 2x 1TB and 1x 2TB drive, you get 2 TB of usable space.

It is arguably not the most useful property in enterprise setting, but helps to stay on budget on home projects, since I can just use random drives laying around without any further requirements on the matching of the sizes.

as far as I know, you can put disks of different sizes in a mirror vdev, you’ll just obviously be limited to the size of the smaller disk.

Is that not what OC is asking for?

Btrfs' RAID1 works bit differently. It ensures every block (1GB by default I believe) is present on two devices. So, if for example, you have 2x 1TB and 1x 2TB drive, you get 2 TB of usable space.

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